There are a number of problems known in the field of optical communications, which concern regulating power of multi-channel optical transmission.
In advanced network configurations, sites can possibly contain either optical amplifiers or Optical Add Drop Multiplexers (OADM), or both. An optical trail may start and terminate at any site according to a customer's use. For example, due to the fact that OADM is capable of dropping and adding optical channels, some trails may start or terminate at the add/drop ports of OADM and the number of optical channels in spans before and after OADM may differ significantly. In such configurations, a fiber cut in a span/section beyond the end points of a specific optical trail must not have any effect on the performance of such an optical trail. Some control systems handling optical transmission in the advanced network configurations take care of the above problem.
Some of the presently used systems for DWDM networks introduce a so-called channel count mechanism. This mechanism transmits the number of active channels in the link to all the amplifiers in that link so they can modify their output power accordingly. In order to maintain balance in such a system, the number of channels dropped at a particular node should be approximately equal to the number of channels added at this node. However, such a mechanism reacts incorrectly to events such as a fiber cut in a specific span of a link with OADMs (Optical Add Drop Multiplexers). Indeed, a fiber cut drastically changes the actual number of channels at more than one successive nodes, so the output power of the amplifiers in the link will not be correctly adjusted. The erroneous adjustment will most likely cause a degradation in optical performance of the system.
U.S. Pat. No. 6,072,601 describes an optical fiber amplifier capable of determining a number of optical channels actually inputted to it, thereby allowing, through self-control, both optimum operation conditions in accordance with the number of transmission channels, and the maintenance of the transmission performance. The optical fiber amplifier comprises an optical signal amplifier and a control circuit. The control circuit section comprises a branch circuit that branches off and extracts a portion of the transmitted optical signal power, and a channel counter that inputs a portion of the branched optical signal power and based on that counts the number of channels of the transmitted optical signals. The control circuit controls the amplification factor of the optical signal amplifier in accordance with the number of channels counted at the channel counter. The system counts the channels on the ad-hoc basis but is unable to predict the number of active optical channels in case of a fiber cut, which would be important to prevent ill effects of transient periods in the network until the channel count mechanism is updated.
WO 9921302 A1 relates to a method for controlling the output power of an optical amplifier in a node in a network, wherein the output power of the amplifier is controlled via a pump laser in a control circuit. The node reads a supervisory channel OSC comprising information on how to control the amplifier, and then uses the information for the control of the amplifier. The mentioned information comprises the number of channels coming into the node. Also, input power to the amplifier is monitored and, if it is changed more than a certain limit, further change of the amplifier control is withhold up to receiving a reliable number of the optical channels.
U.S. Pat. No. 6,275,329 describes a method and apparatus for amplifying an optical signal in an optical network. The optical amplifying module comprises a variable optical attenuator (VOA), an optical amplifier and a controller, which are coupled to each other and to the optical network. The VOA attenuates the optical input signal to produce an attenuated output signal. The optical amplifier amplifies the latter signal to produce an amplified output signal. The controller receives some network operating parameters and also monitors the power levels of the attenuated and the amplified signals. Based on the measured power levels and the received operating parameters the controller adjusts the VOA such that the optical output power per channel is maintained at a constant value.
Neither of the presently known systems allows simply and suitably adapting power control in an optical network to different types of faults, for example reacting adequately to a fiber cut and/or a fiber degradation due to fiber bend or fiber aging.
Modern networks, which are to satisfy requirements of the standards accepted in the field, should provide fast reactions to faults in order to ensure the traffic non-effecting operation. For example, a fiber cut in a telecommunication link should be detected and isolated during no more than 50 ms.
Presently, there is no known methods/systems that simply and operatively detect faults and adjust power in an optical communication system so, that occurrence of a fiber degradation would not lead to the system unbalancing.
A co-pending Applicant's patent application U.S.-2003-0046293-A1 (WO 03/021835 A2) describes a method of real time control of power per optical channel in a multi-channel optical communication line formed by a group of optical elements connected in a chain by fiber spans, the method comprises calculating, for a particular optical amplifier in the line, an expected total input power value EIP, measuring a real total input power MIP at said particular optical amplifier, and, if a difference between the EIP and the MIP exceeds a predetermined value, adjusting the amplifier's gain to maintain its output power per channel constant. The EIP value is calculated using up-to date values of NOC (number of active optical channels) and NOA (number of preceding optical amplifiers in the line). That solution requires preliminary calculations for obtaining NOC, NOA, EIP for providing a reaction to changes in the line.
The mentioned above documents are hereby incorporated by reference.